Use extra speakers with my avt 100 combo

The ohms output on my Marshall AVT100 Combo says 8ohms,(on the electrical circuit diag,shows a connection for 4 ohms output,not used),the other guitarist in the band uses a Marshall 100Wts Head and a 4 x 12 cab,my AVT doesnt seem half as loud,,I belive I will get more wattage,perhaps double.

Question, Can I use a 4ohm Cab with out damaging the amplifier.(obviously using speakers with double the Wattage rating required,eg,200+ etc). I hope some one can help.thanks.Chalkydave.